function f1() {
	var username = document.getElementById('username');
	var a = document.getElementById('username_span');
	a.innerHTML = "输入正确";
	if (username.value.length == 0) {
		a.innerHTML = "账号不能为空";
		return false;
	}
	return true;
};
function f2() {
	var password = document.getElementById('password');
	var b = document.getElementById('password_span');
	b.innerHTML = "输入正确";
	if (password.value.length == 0) {
		b.innerHTML = "请输入密码";
		return false;
	}
	return true;
};

window.onload=function(){
	var next=document.getElementById('next');
	next.onclick=function(){
		document.getElementById("code").src="code?"+Math.random();
	}
}